1. Field of the Invention
The present invention relates to the design of passive optical networks. More specifically, the present invention relates to a method and apparatus for dynamically allocating upstream bandwidth in a passive optical network.
2. Related Art
In order to keep pace with increasing Internet traffic, optical fibers and associated optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this increase in the capacity of backbone networks has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among different technologies, Ethernet passive optical networks (EPONs) appear to be the best candidate for next-generation access networks. EPONs combine the ubiquitous Ethernet technology with inexpensive passive optics. Therefore, they offer the simplicity and scalability of Ethernet, and the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, since Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. Logically, the first mile is a point-to-multipoint network, with a central office servicing a number of subscribers. A tree topology can be used in an EPON, wherein one fiber couples the central office to a passive optical splitter, which divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers (see FIG. 1).
Transmissions within an EPON are typically performed between an optical line terminal (OLT) and optical networks units (ONUs) (see FIG. 2). The OLT generally resides in the central office and couples the optical access network to the metro backbone, which is typically an external network belonging to an ISP or a local exchange carrier. The ONU can be located either at the curb or at an end-user location, and can provide broadband voice, data, and video services.
Communications within an EPON can be divided into upstream traffic (from ONUs to OLT) and downstream traffic (from OLT to ONUs). Because of the broadcast nature of Ethernet, the downstream traffic can be delivered with considerable simplicity in an EPON: packets are broadcast by the OLT and extracted by their destination ONU based on their media access control (MAC) addresses. However, in the upstream direction, the ONUs need to share the channel capacity and resources. Moreover, the burstiness of network traffic and the requirement of different service level agreements (SLAs) make the upstream bandwidth allocation a challenging problem.
Hence, what is needed is a method and apparatus for dynamically allocating upstream bandwidth in an EPON, which is fair, efficient, and responsive, and which accommodates bursty traffic while satisfying SLAs.